Burn, Baby, Burn

Ever since humankind learned to make fire, people have relied on burning things – to keep warm, cook our food, see at night, and (nowadays) to power our vehicles and charge our cell phones. Our world relies heavily on a process scientists refer to as incomplete combustion, which refers to the burning of biological materials such as wood or fossil fuels, to harness energy. Unfortunately, our appetite for energy comes at a high cost—we now know that fossil fuel combustion releases carbon dioxide (CO2), a greenhouse gas, into the atmosphere, where it warms our planet, alters the chemistry of our oceans, and perturbs the global carbon cycle.

Just as charred wood is left behind after your campfire has burnt out, chemical remnants besides CO¬2, such as PAHs, are formed as byproducts when we burn fuels incompletely. Source: Kai Schreiber via Flickr, used under Creative Commons license

While we all know that CO2 is a harmful byproduct of burning fuel, it’s not the only chemical formed during this process. CO2 forms when complete combustion of organic material occurs – there are many other chemicals that are also formed during incomplete combustion. Just as charred pieces of wood are left behind after your campfire has cooled, chemical remnants are left behind when we burn wood, petroleum, coal, or even when we char a steak on the barbeque. We’ve been releasing these byproducts into the environment in increasingly large amounts as we’ve built more cities, driven more cars, and demanded more and more electricity, but we don’t know much about their effects on the cycling of carbon through our air and oceans.

Polycyclic aromatic hydrocarbons, or PAHs, are one class of chemical released whenever organic material is burnt. PAHs can come in a wide variety of shapes and sizes, but all feature the same building block—the aromatic benzene ring. PAHs are also released in large amounts episodically when oil spills occur, and they’re produced as a result of natural events like forest fires and volcanic eruptions. Natural or not, PAHs aren’t harmless –they’re considered the main cancer-causing component of urban air pollution.

Like humans, animals, and other organic matter, PAHs are mostly made from carbon, the currency of all life on Earth. When we burn carbon for energy, we don’t destroy the actual carbon – the chemical byproducts of combustion become vehicles delivering that carbon back into global reservoirs, including the ocean. Carbon dioxide is the main compound perturbing global cycles because such a large amount of it is being produced compared to other compounds – PAHs released from human activities are usually considered minor components of carbon we’re releasing into the air. However, they may play a bigger and more complex role in the chemistry of our world than we had previously thought. In this study, researchers traveled the world measuring PAHs in air and water samples from three different ocean basins to learn more about the PAHs we’re depositing into the oceans.

Circumnavigation for Science

The Malaspina 2010 Circumnavigation Expedition was an ambitious voyage involving hundreds of researchers from around the world, whose purpose to collect tens of thousands of precious air, water, and biological samples from the world’s oceans. The expedition provided a unique opportunity for researchers to conduct extensive air and water sampling across multiple ocean basins.

In this study, scientists measured 64 different PAHs in air and seawater samples from the Atlantic, Pacific, and Indian Oceans. By measuring the levels of these chemicals in air and water, they could calculate the rates at which PAHs were being deposited into the ocean’s surface from the air. However, they realized that measuring each chemical individually wasn’t going to give them a good idea of how the thousands of chemicals in their samples might be impacting the Earth’s chemistry, so they also developed a way to extract the aromatic fraction of their samples, which contained PAHs as well as other related compounds, and measured the total amount of all of these compounds in each sample as a lump sum.

The graph above shows signals (peaks) produced when PAHs are related compounds are detected using a gas-chromatograph/mass spectrometer (GC/MS). The red line shows the peaks found when researchers only looked for 64 known PAHs, and the blue line also includes all of the related compounds in the sample that the researchers didn’t specifically target. Reprinted by permission from Macmillan Publishers Ltd: Nature Geoscience (Gonzalez-Gaya et al.), copyright 2016.

How Do PAHs End Up in the Oceans?

Researchers in this study collected samples to investigate three different ways that PAHs can be delivered to ocean waters, namely, dry deposition, wet deposition, and gaseous absorption. Dry deposition refers to the settling of particles from the air into surface water – much like how ash can be blown from your campfire and land far away, PAHs are often glommed together on very small particles that can travel great distances before finally depositing. Wet deposition is a similar process, but occurs when it rains and many particles are scavenged from the air and deposited in the ocean. Gaseous absorption, usually considered to be a much more minor input route, occurs when individual PAHs in the gas-phase (not glommed to particles) are absorbed into surface waters from the air via diffusion.

Fluxes of PAHs (in tonnes per month) delivered to the surface oceans via gaseous absorption (top plot, FAW) and dry deposition (bottom plot, FDD). Dry deposition tends to deliver more of the PAHs on the right of the plot, which are larger molecules, because they more readily glom onto particles in the air. Gaseous absorption delivers more of the smaller PAH molecules into the water, and is responsible for a much larger input of PAHs into the oceans than was expected. Positive bars in the absorption plot represent volatilization (loss of molecules escaping from the water’s surface into the air), while negative bars represent absorption. Reprinted by permission from MacMillan Publishers Ltd: Nature Geoscience (Gonzalez-Gaya et al.), copyright 2016.

Usually, dry deposition is considered the major route for PAHs to enter surface waters, but this study found that absorption was actually more important, resulting in 90 times greater PAH inputs than dry deposition. Total global inputs from gaseous absorption during one month were 4 times greater than the total amount of PAHs estimated to have entered the water during Deepwater Horizon.

Granted, the numbers presented in this study are for PAHs are being deposited over the whole globe rather than in one region as in an oil spill, but the study drives home the point that this is not a trivial source of carbon; especially when we consider that these numbers were estimated using only data from the targeted PAHs measured in this study. The total amount of related compounds being deposited could be 100-1000 times greater than what the researchers were able to directly measure.

The authors estimate that global deposition of PAHs and related aromatic compounds results in inputs of carbon around 15% of those from CO2, which highlights these compounds as important targets of further study – what effects might they be having on our oceans and the life within them?

What Now?

Scientists are still only scratching the surface of what this study’s findings mean for the health of our oceans. The authors raise some interesting questions based on their results; two of the main ones were:

Might PAHs be affecting the bacterial composition of the oceans, or might continuing inputs of PAHs “train” those bacteria to become better at degrading these chemicals? PAHs are naturally degraded by bacteria in seawater. For this reason, our steady inputs of PAHs into ocean waters over decades could be an important source of food helping certain bacterial species in thrive. Our prolonged inputs of PAHs into surface waters in remote open ocean waters where food is scarce may even have helped to condition bacteria to become adept at breaking down PAHs – something we know they are capable of doing in the event of an oil spill. Have we been training bacteria to clean up our messes?

What are PAHs doing to marine wildlife? Some PAHs can bioaccumulate, meaning that as we produce more and more of these chemicals, they will end up in living things, such as fish and marine mammals, at increasing levels. This is worrisome because we know some PAHs cause cancer, and they could cause serious consequences for the health of ocean life that we aren’t yet able to measure or understand.

More on PAHs

Want to learn more about PAHs in the ocean? Check out these past Oceanbites posts:

I am the founder of oceanbites, and a postdoctoral fellow in the Higgins Lab at Colorado School of Mines, where I study poly- and perfluorinated chemicals. I got my Ph.D. in the Lohmann Lab at the University of Rhode Island Graduate School of Oceanography, where my research focused on how toxic chemicals like flame retardants end up in our lakes and oceans. Before graduate school, I earned a B.Sc. in chemistry from MIT and spent two years in environmental consulting. When I’m not doing chemistry in the lab, I’m doing chemistry at home (brewing beer).